This invention relates to the fracturing of subterranean formations and more particularly relates to a method for forming a horizontally disposed fracture in a subterranean formation by the combined application of hydraulic pressure and acoustical energy.
Methods of fracturing subterranean formations penetrated by boreholes by the application of hydraulic pressure are well known. It is generally accepted that at depths greater than about 2000 to 3000 feet vertical fractures are normally preferentially formed in subterranean formations rather than horizontal fractures by hydraulic fracturing techniques.
Methods of fracturing subterranean formations by the combined application of hydraulic pressure and acoustical energy are also shown in the prior art. In U.S. Pat. No. 2,915,122 there is described a method for hydraulically fracturing a subterranean formation penetrated by a wellbore wherein a plurality of mechanically induced pressure shocks are applied to a low penetrating fluid that is pumped via a wellhead into the wellbore. The pressure shocks are applied directly to the column of liquid at the wellhead by means of an air hammer or a piston or other suitable apparatus. The pressure shocks may be applied to the column of liquid as the pressure of the liquid is increased to the formation breakdown pressure, or may be applied when the formation breakdown pressure is reached and fissures are formed in the formation or may be applied continuously as the fluid pressure increase is begun and throughout the entire period of pressure application. In U.S. Pat. No. 3,602,311 there is described a process for fracturing a subsurface formation wherein a first path of flow of a fracturing fluid is established from a pump through a flow control-hammer valve and a second path of flow is established to the formation to be fractured from a point between the pump and the valve, the pressure of the fluid against the formation from the second path of flow being controlled at a predetermined level, preferably slightly less than the formation fracture pressure. Next, the flow through the valve is instantaneously terminated to cause a pressure pulse to be transmitted through the second path of flow and against the formation and to cause fracturing thereof.
In U.S. Pat. No. 3,842,907 there is disclosed an acoustical well fracturing method and apparatus whereby pressure fluctuations are generated in a wellbore by pumping fluid through a first conduit to drive an acoustical oscillator coupled with an acoustical compliance that transmits the pressure fluctuations to a formation of the earth in a selected zone of the wellbore. The wellbore which functions as a second conduit that contains the first conduit and the oscillator returns fluid flow back toward a pump means. A variable restriction means is used to adjust the back pressure in the wellbore such that the maximum oscillated fluid pressure exceeds that pressure required for a formation fracture. To achieve fracture only in the selected zone, acoustical isolation means are spaced above and below the acoustical oscillator to confine the pressure fluctuations to that zone.
An article by Ralph O. Kehle entitled "The Determination of Tectonic Stresses through Analysis of Hydraulic Well Fracturing," Journal of Geophysical Research, Vol. 69, No. 2, Jan. 15, 1964, pp. 259-273, considers the problem of the determination of the magnitudes of the components of the tensile stress at a point in the earth's crust. This determination is made using data that is normally obtained during a hydraulic well-fracturing operation. The well-fracturing operation is modelled by a band of uniform pressure and two bands of uniform shear stress acting in a cylindrical cavity in an infinite body. The model is an open hole well-fracturing treatment wherein the prospective producing horizon is packed off and fluid is introduced into this zone. The fluid pressure is increased to a maximum value at which time the formation fractures. Fluid continues to be pumped into the zone, and the pressure stabilizes at a value, the flowing pressure, which commonly is intermediate to the formation fluid pressure and the breakdown pressure. The fracturing fluid induces a uniform pressure over the packed-off interval. The effect is modelled exactly by a uniform band of pressure in a cylindrical hole. The pressure also tends to force the packers away from the pressurized interval, but any such movement of the packers is prohibited by frictional forces that arise at the contact between the packers and the wall of the borehole.
It is stated that two interesting regions of induced stress are: either end of the pressurized interval where the tangential stress is zero (the vertical stress is approximately 95 percent of the pressure) and the center of the packed-off interval where the tangential stress equals the pressure (the vertical stress is zero). The tectonic stresses are the overburden load and two unknown principal horizontal stresses that cause easily determined stress concentrations at the wellbore. All calculated stresses are modified to account for the interstitial pore-fluid pressure. It is found that three situations are of interest: (1) the induced vertical stress is less than the overburden pressure; (2) the induced vertical stress and the instantaneous shut-in pressure are greater than the overburden pressure; (3) the induced vertical stress is greater than the overburden pressure, but the instantaneous shut-in pressure is less than the overburden pressure. In (1) the fracture is vertical and the stresses are determinable. In (2) the fracture is horizontal and the stresses are indeterminable. In (3) the fracture is initially horizontal but becomes vertical as it propagates from the well, the vertical and minimum horizontal compressions are determinable, and the other principal stress is bounded by a set of inequalities.
In another article by H. von Schonfeldt and C. Fairhurst, entitled "Open Hole Hydraulic Fracturing," Third Symposium on Salt, Volume Two, The Northern Ohio Geological Society, Inc., Cleveland, Ohio, pp. 404-409, the basic principles of hydraulic fracturing in an open hole are discussed. The pre-existing regional stresses produce stress concentrations close to the borehole with a maximum value and a minimum value in tangential directions and at right angles one to the other. If a section of the borehole is sealed or packed off and pressurized, a second stress system will be superimposed upon the one just described. Two regions in the sealed interval to be considered are the central part of the hole section and the region close to the packers. It is stated that according to Kehle's model we arrive by superposition of the two active stress systems in the packed-off borehole section at certain expressions for the net minimum pressure (least compression). Expressions are then derived for the tangential stress in the central region and the axial stress in the region close to the packers. It is stated that a fracture is assumed to initiate at whichever point the appropriate strength of the rock is first reached. Under these assumptions expressions are then derived for the pressures which result in (1) a fracture parallel to the borehole axis (vertical fractures), and (2) a fracture normal to the borehole axis (horizontal fracture). It is then stated that if pressurized, deformable packers are used, Kehle's model will no longer be valid in the vicinity of the packers. This means that little or no axial tension is generated, thus requiring a much higher pressure for the initiation of horizontal fractures. Before such a high pressure is reached, however, a fracture will be initiated vertically.